The present invention relates to a disk storage medium on which data is recorded by light (which will be referred to as an “optical disk”).
In recent years, optical disks such as DVD-RAM and DVD-RW have been used as storage media for recording digital information thereon at a high density. Each of these optical disks used commonly today is designed in such a manner as to record data of 4.7 GB per side by being irradiated with a laser beam having a wavelength of 650 nm through an optical system (e.g., objective lens) having a numerical aperture of 0.6. Thus, approximately one hour of video signal can be recorded on each side.
However, the maximum recordable length of approximately one hour is not long enough to cope with most of actual applications. Accordingly, to make those optical disks as handy as home video tape recorders, those optical disks should acquire an even greater storage capacity. Also, to perform editing and other types of operations by making full use of the random-access capability, which is one of advantageous features of the optical disks, video signal needs to be recorded for about five hours or more. In that case, the data storage capacity of the optical disks should be at least 23 GB and preferably more.
However, it is not easy to produce an optical disk with such a huge capacity because the recording density must be tremendously increased from the currently available one.
The present invention overcomes the problems described above, and a primary object thereof is to provide an optical disk that achieves a high recording density and a huge storage capacity.